(a) Field of the Invention
This invention relates to a fine-particle aggregate emulsion useful as an additive for coating compositions used as paints and in paper coating and information recording paper, as well as a process for producing the same.
(b) Description of the Prior Art
In recent years, various particulate polymers have been being studied in order to develop organic pigments useful as additives for coating compositions.
The most commonly used particulate polymer is homogeneous, emulsion-polymerized polystyrene particles having a diameter of 0.2 to 0.5 .mu.. For example, Japanese Patent Laid-Open No. 59741/'84 discloses a process wherein an unsaturated carboxylic acid and a vinyl monomer are copolymerized in the presence of an anionic surface active agent and/or a nonionic surface active agent to form a copolymer emulsion in which more than 90% of the particles have a diameter of 0.20 to 0.28 .mu.. It is also described therein that this copolymer emulsion can be used in applications including paper coating, paints and the like. However, the organic pigment produced by this process comprises particles having a uniform composition throughout, so that it fails to provide sufficient hiding power and brightness. Accordingly, no practical advantage is obtained unless it is used in large amounts.
In recent years, an organic pigment composed of particle having pores, unlike homogeneous and solid particle, has been proposed in order to further improve hiding power and brightness (U.S. Pat. No. 4,427,836). This patent discloses a process for producing aqueous dispersion which comprises providing a dispersion of core particles formed from a polymer containing at least 5% of a carboxylic acid monomer, adding thereto at least one monoethylenically unsaturated sheath monomer for forming a sheath polymer, subjecting this monomer to emulsion polymerization, and neutralizing the resulting emulsion with an aqueous volatile base to swell the core particles and thereby form minute openings therein.
When the organic pigment produced by this process is used in paints or paper coating compositions, an improvement in hiding power and brightness is achieved as compared with the use of organic pigments composed of homogeneous and solid particles. However, this organic pigment has the disadvantage that, according to the usage, its pores may be destroyed to cause a loss in hiding power and brightness. The pores are destroyed by freezing, some solvents used in paint formulations, and heating. Thus, this organic pigment can be used only under limitations on formulation and usage.